Digital optical discs (referred to as DVD discs) are commonly used to store software applications and content in digital form, typically 4.7 Gigabytes per disc or more. DVD discs come in a variety of formats: manufactured discs, such as DVD-ROM, one time recordable discs, such as DVD-R, and rewritable discs such as DVD-RAM and DVD−RW. DVD-ROM discs are generally mass-produced in a disc-replication manufacturing process that includes glass mastering, electroforming, and stamping. Recordable and rewritable discs are generally produced individually or in small quantities by DVD-recorders containing suitable blank media. DVD-recorders and blank media, together with appropriate “burning” software, are readily accessible to individual consumers.
DVD discs are susceptible to unauthorized replication. Due to the digital nature of DVD, such an unauthorized discs may be essentially identical to authentic ones. Unauthorized discs can be manufactured on a mass-produced basis, or can be recorded by consumers with a computer, DVD-recorder, and blank media. The problem of unauthorized end user recording is growing more acute, now that DVD-recorders are available for a few hundred dollars, and blank media is available for around 5 dollars.
Digital data written to digital optical discs (DVD) undergoes a well-defined series of processing steps including interleaving, Reed-Solomon encoding, and Eight-to-Sixteen encoding (known as EFM-Plus), for DVDs.
EFM+ encodings are physically represented by microscopic patterns of pits and lands (or in the case of rewritable discs, by microscopic regions of lesser and greater reflectivity). An encoded ‘1’ represents a transition between a pit and land (or lesser and greater reflectivity), while an encoded ‘0’ represents a continuation of a pit or land ( or a continuation of lesser or greater reflectivity).
Excessive physical damage to the media surface, such as a scratch, can obscure the precise location of the transitions and thereby corrupt the data reading. To protect against such hazards, the media is written with additional data in the form of error correcting symbols. These are mathematically determined to correspond to the other data written on the media in such a way that the player can use them as it reads the media not only to determine if errors have occurred, but under certain conditions to correct errors. A fixed number of data symbols, together with fixed number of symbols compacted for them form a data structure known as an error correction codeword (ECC). Methods for choosing and implementing appropriate error correction coding are well-known in the art.
In ordinary use, error detection and correction is desirable, as it reduces the probability that the data read from digital optical media and delivered to the user will become corrupted by minor physical damage to the media.
However, making changes to the disc in order to prevent the disc from being illegally copied are likely to be frustrated by the ECC of the DVD.